diff --git a/milestones/README.md b/milestones/README.md
index 3b084ef..a59b7c2 100644
--- a/milestones/README.md
+++ b/milestones/README.md
@@ -5,11 +5,13 @@ repositories that install `docgen` and maintain their own demo bundle. The
 library no longer ships an in-repo dogfood; consumers are the integration test
 of record.
 
-**Active:** **[image-generation-strings.md](image-generation-strings.md)** —
-`image_generation.model` / `size` / `quality` must be YAML strings at
-config load.
+**Active:** **[ai-timestamp-strings.md](ai-timestamp-strings.md)** —
+`ai.provider` / `timestamps.engine` / `tts.language` must be YAML strings
+at config load.
 
 **Shipped:**
+- **[image-generation-strings.md](image-generation-strings.md)** —
+  `image_generation.model` / `size` / `quality` must be YAML strings (#104).
 - **[image-empty-bytes.md](image-empty-bytes.md)** —
   `image-generate` must not write empty PNG bytes as success (#103).
 - **[hint-front-matter-yaml.md](hint-front-matter-yaml.md)** —
diff --git a/milestones/ai-timestamp-strings.md b/milestones/ai-timestamp-strings.md
new file mode 100644
index 0000000..304dace
--- /dev/null
+++ b/milestones/ai-timestamp-strings.md
@@ -0,0 +1,40 @@
+# Milestone: leftover AI / timestamps / TTS language keys must be strings
+
+**Status:** Active  
+**PR:** [#105](https://github.com/jmjava/documentation-generator/pull/105)  
+**Depends on:** `milestones/image-generation-strings.md` (PR #104),
+`milestones/tts-empty-audio.md` (PR #99),
+`milestones/multi-host-ai-hardening.md`
+
+## Problem
+
+`tts.model` / `image_generation.model` are already strings at config load.
+These sibling keys were not:
+
+1. **`ai.provider` / `base_url` / `api_key_env`** — a YAML list was
+   `str()`’d (`"['openai']"`) into `normalize_provider`.
+2. **`timestamps.engine`** — a list became `"['local']"`, then
+   `resolve_engine` raised unknown-engine only when timestamps ran.
+3. **`tts.language`** — a list was `str()`’d into xAI TTS/STT
+   `language`.
+
+## Goal
+
+Fail closed at `Config.from_yaml`. Missing keys still use defaults
+(`ai.provider` openai, `timestamps.engine` local, Grok language `en`).
+
+## Done when
+
+- [x] Present `ai.provider` / `base_url` / `api_key_env` must be
+      non-empty YAML strings.
+- [x] Present `timestamps.engine` must be a non-empty YAML string.
+- [x] Present `tts.language` must be a non-empty YAML string.
+- [x] Tests for list values of those keys.
+- [x] `ruff check src/ tests/`
+- [x] `pytest tests/`
+- [x] `docgen benchmark` (no clock change)
+
+## Out of scope
+
+- Unknown `timestamps.engine` values still fail at `resolve_engine`.
+- `manim.font` / `manim.quality` string typing is separate.
diff --git a/milestones/image-generation-strings.md b/milestones/image-generation-strings.md
index dd34669..c40fb7d 100644
--- a/milestones/image-generation-strings.md
+++ b/milestones/image-generation-strings.md
@@ -1,7 +1,7 @@
 # Milestone: image_generation.model / size / quality must be strings
 
-**Status:** Active  
-**PR:** pending  
+**Status:** Shipped  
+**PR:** #104  
 **Depends on:** `milestones/image-empty-bytes.md` (PR #103),
 `milestones/tts-empty-audio.md` (PR #99)
 
diff --git a/src/docgen/config.py b/src/docgen/config.py
index 68277ad..726cd40 100644
--- a/src/docgen/config.py
+++ b/src/docgen/config.py
@@ -289,6 +289,18 @@ def __post_init__(self) -> None:
             require_yaml_string(ig["size"], label="image_generation.size", source=src)
         if ig.get("quality") is not None:
             require_yaml_string(ig["quality"], label="image_generation.quality", source=src)
+        ai = self._block("ai")
+        if ai.get("provider") is not None:
+            require_yaml_string(ai["provider"], label="ai.provider", source=src)
+        if ai.get("base_url") is not None:
+            require_yaml_string(ai["base_url"], label="ai.base_url", source=src)
+        if ai.get("api_key_env") is not None:
+            require_yaml_string(ai["api_key_env"], label="ai.api_key_env", source=src)
+        ts = self._block("timestamps")
+        if ts.get("engine") is not None:
+            require_yaml_string(ts["engine"], label="timestamps.engine", source=src)
+        if tts.get("language") is not None:
+            require_yaml_string(tts["language"], label="tts.language", source=src)
         ocr = self._sub_block(validation, "ocr", label="validation.ocr")
         if ocr.get("error_patterns") is not None:
             string_list_block(
diff --git a/tests/test_config.py b/tests/test_config.py
index 36f0e20..30fd077 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -486,3 +486,31 @@ def test_from_yaml_list_image_generation_quality_raises(tmp_path: Path) -> None:
     p.write_text("image_generation:\n  quality:\n    - high\n", encoding="utf-8")
     with pytest.raises(ConfigError, match="image_generation.quality must be a YAML string"):
         Config.from_yaml(p)
+
+
+def test_from_yaml_list_ai_provider_raises(tmp_path: Path) -> None:
+    p = tmp_path / "docgen.yaml"
+    p.write_text("ai:\n  provider:\n    - openai\n", encoding="utf-8")
+    with pytest.raises(ConfigError, match="ai.provider must be a YAML string"):
+        Config.from_yaml(p)
+
+
+def test_from_yaml_list_ai_base_url_raises(tmp_path: Path) -> None:
+    p = tmp_path / "docgen.yaml"
+    p.write_text("ai:\n  base_url:\n    - https://api.openai.com/v1\n", encoding="utf-8")
+    with pytest.raises(ConfigError, match="ai.base_url must be a YAML string"):
+        Config.from_yaml(p)
+
+
+def test_from_yaml_list_timestamps_engine_raises(tmp_path: Path) -> None:
+    p = tmp_path / "docgen.yaml"
+    p.write_text("timestamps:\n  engine:\n    - local\n", encoding="utf-8")
+    with pytest.raises(ConfigError, match="timestamps.engine must be a YAML string"):
+        Config.from_yaml(p)
+
+
+def test_from_yaml_list_tts_language_raises(tmp_path: Path) -> None:
+    p = tmp_path / "docgen.yaml"
+    p.write_text("tts:\n  language:\n    - en\n", encoding="utf-8")
+    with pytest.raises(ConfigError, match="tts.language must be a YAML string"):
+        Config.from_yaml(p)